Rice advised Trump not to negotiate any details with Kim during their meeting, and added that other countries like Japan and South Korea also have interest in the summit and its potential outcomes.

"Really recognize that others are at stake," she said, adding that Pyongyang has a history of entering agreements and then breaking their promises.

Second, she recommended that Trump not be anxious about moving American military forces on the Korean Peninsula.

She also urged the president not to forget the true nature of Kim's regime, labeling it as "brutal."

"Human rights violations, a death camp for its own citizens. Don't forget the nature of the regime," she said.

Rice also downplayed the importance of where the meeting will be held, as there have been discussions of holding it near the Korean Demilitarized Zone.

"It just has to be very well-prepared," she said.

The president tweeted that the planning for his meeting with Kim is "going very well" after he spoke with South Korean President Moon Jae-in on Saturday.

In a historic summit, Moon and Kim met Friday and vowed to denuclearize the Korean Peninsula.

Just had a long and very good talk with President Moon of South Korea. Things are going very well, time and location of meeting with North Korea is being set. Also spoke to Prime Minister Abe of Japan to inform him of the ongoing negotiations.